...i recon temp is far more of a problem than the amount of light for many leafy things and a few extra degrees with a very low risk of freezing gives a four season salad bowl if you chose the hardier varieties....

Careful with that if you're concerned about nitrate levels in your foodstuffs

the manure goes in a deep trench ,the salad roots are in the normal soil above it so they feed as usual.if planted strait into manure they could be a bit nitratey if the roots survived.the manure is to provide a bit of extra warmth as it rots.i spose if one was very worried the manure could be separated from the grow media by a sheet of plastic which could be pulled out later.

It's less about N supply, and more about lack of light. In deep winter with short days there isn't enough photosynthate (non-structural carbohydrates) being produced for the plant to regulate it's osmotic pressures the way it wants to. (especially with temperature stresses) so it shuts down nitrate assimilation, and keeps nitrates around for osmotic regulation. (also there's less solar energy to power the nitrate assimilation and provide carbon for the nitrogen to be paired with)

interesting,i wonder if that applies to winter forage of leafy greenstuffs ?i suspect tree leaves are powered by stored carbs and until bud burst and are less of a problem than overwintering stuff or very early sprouters.

im beginning to wonder about nitrate levels in sap as well.

i spose if one is reliant on home grown or forage the extra nitrate is less of a worry than a lack of vit c but it is worth considering as a potential problem .

As far as sap and young leaves are concerned, people have been using those as food and drink for hundreds or thousands of years and survived. I know they tended to die younger, but not usually of cancer which is what you would expect from high nitrates.